Infection by enveloped viruses is initiated by binding of the viral envelope glycoprotein(s) to specific receptor molecules on the target cell, followed by fusion between the viral and cellular membranes. For the morbilliviruses measles virus (MV), and canine distemper virus (CDV), fusion requires co-expression of the virus fusion (F) and hemagglutinin (HA) glycoproteins on the surface of the virus, as well as the presence of the appropriate receptors on the target cell. In previous work, we showed using cell-cell fusion assays that fusion occurs efficiently with heterologous expression of F from one virus and H from the other; cell-type specificity is determined by H. We also obtained direct evidence for functional and structural interaction between measles H and CD46, the known MV receptor